The effects of salinity on soil have great ecological and economic effects. Determining salinity levels of soil is useful to identify areas to avoid developing and/or rehabilitate. A convenient mechanism for surveying large areas of land is synthetic aperture radar (SAR). SAR data can be collected from aircraft or from spacecraft. Polarimetric SAR has the ability to be acquired under cloudy conditions and is sensitive to the dielectric constant of the soil. It is known that the dielectric constant (∈) is indicative of the salinity level of the soil. Electrical conductivity is another indicator of the salinity level of soil. The dielectric constant influences the return signal of a radar beam.
The dielectric constant is a complex number. The real part of the dielectric constant ((∈)) is known as the permittivity and the imaginary part ((∈)) is known as loss factor. The real component is a good discriminator for moisture level of soil, while the imaginary component is used as a discriminator for soil salt content. It is known to be able to determine the dielectric constant from a scattering matrix containing information on the scattering intensity from different polarisations, phase shifts and correlation co-efficient data of the return signal of polarimetric synthetic aperture radar. The polarisations are based on the transmit/receive orientations of the electromagnetic wave. All bands (frequencies) are transmitted and received in horizontal (H) and vertical (V) orientations. Thus the recorded data includes the power level returned for orientations H-H, H-V, V-H and V-V for each frequency. A dielectric constant inversion algorithm may be used to convert the polarimetric SAR data to provide dielectric values which can then in turn be used to determine salinity levels.
Often the area scanned by the SAR scans are covered with vegetation. In areas with no cover or semi-vegetation the abovementioned technique is relatively robust. However sometimes areas are covered in significant vegetation, which can have an effect on the return radar signal and thus the data collected. In particular the signal is attenuated or has double bounce characteristics which are not conducive to inversion using the abovementioned techniques.
As a precursor to the present invention a technique suggested by Taylor et al in Characterisation of saline soils using airborne radar imagery, Remote Sens. Environ. (1996), 57, 127–142, improved the estimation of the dielectric constant received from the SAR data. This technique improved the accuracy of some types of ground cover but not others.
An object of the present invention is to provide improved salinity results based on radar data for a greater range of ground cover types.